I read that most straight hairline cracks are pretty normal, especially around windows or doors, due to settling and stress. Mine happens below a pretty wide extruding window and is not perfectly straight. It has a short horizontal region before turning vertical. The width of the crack is less than 1mm (1/25 inch).
Is this something of concern?
Is there any downside to leaving the crack as is behind baseboard heater?
My rough plan is to repair it with tape and drywall mud for the visible portion.